In the conventional mobile communication, a poor radio channel environment causes decreased reliability or time fluctuation of radio channels, and channel encoding and error correction techniques are therefore required. Currently, a hybrid automatic repeat request (HARQ) technique is a relatively common technique that combines automatic repeat request (ARQ) technique and forward error correction (FEC) technique are combined, thereby performing error detection and correction. Hybrid automatic repeat request techniques are presently classified in three types. Type I discards a packet that is not received correctly on the receiving side, makes the transmission side retransmit a copy of the packet by returning a negative acknowledgement (NACK: error), and decodes the newly received packet independently. Type II does not discard the packet resulted in an error on the receiving side and performs decoding by synthesizing it with retransmitted information. Type III is able to synthesize retransmitted information with a packet transmitted before, and the retransmitted packet includes all information necessary for receiving data correctly.
When channel error correction is performed using HARQ, the transmission side first transmits the coded information to the receiving side, and the receiving side receives the information and then performs error correction decoding on the information. When data is received correctly, the receiving side receives the information and, at the same time, transmits an acknowledgement (ACK: no error) to the transmission side, and, when an error is not corrected, the receiving side transmits a negative acknowledgement (NACK: error) and requests data retransmission for the transmission side, and the receiving side performs decoding based on the received retransmission data.
FIG. 1 shows a stop-and-wait ARQ retransmission scheme. With this method, the transmission side transmits protocol data unit (PDU) and then waits until the receiving side returns acknowledgment information. The receiving side returns ACK information upon receiving the data unit correctly. The transmission side receives the ACK information and then transmits the next PDU. The receiving side returns NACK information upon receiving the data incorrectly, and the transmission side receives the NACK information and then retransmits the PDU. Although this method is easy to implement, it is inefficient because the transmission side needs to wait until acknowledgment information is returned.
FIG. 2 shows a go-back-N automatic repeat ARQ retransmission scheme. With this method, the transmission side does not wait until acknowledgment information is returned from the receiving side, and transmits N PDU's sequentially. When the receiving side cannot receive a PDU correctly, the transmission side retransmits the PDU that resulted in an error and subsequent N-1 PDU's that have been transmitted. At the same time, the receiving side discards the PDU that resulted in an error and the subsequent N-1 PDU's. This method has a large round trip delay, and, when an error occurs in one PDU, N PDU's have to be retransmitted, and, consequently, it is hardly efficient.
FIG. 3 shows a selective-repeat ARQ retransmission scheme. With this method, PDU's on the transmission side are similarly transmitted sequentially. However, when an error occurs, only a PDU that resulted in an error is retransmitted. Then, after retransmission, the transmission side continues to transmit a new PDU. This method is efficient but requires reordering of PDU's when they are transmitted to users, and a buffer needs to be provided on the receiving side for storing PDU's that are not resulted in an error, after the PDU that resulted in an error.
FIG. 4 is a flowchart of the conventional ARQ operations. In FIG. 4, when the steps of starting transmission start operating (S400), the receiving side transmits a new signal or retransmits a signal based on the setting (S401). The receiving side starts receiving start operating (S402), receives a signal transmitted from the transmission side (S403), and determines whether or not the received signal has an error (S404). When there is no error, an ACK feedback signal is generated (S405), and, when there is an error, an NACK feedback signal is generated (S406). Then, the generated feedback information is transmitted to the transmission side (S407), and the receiving side reenters the state of starting reception (S408). The transmission side receives the feedback information and then determines whether the received signal is an ACK signal or a NACK signal (S409). If the received signal is an ACK signal, new data is set as transmission content (S410), and, if the received signal is not an ACK signal, the transmitted signal is set as retransmission data (S411). The transmission side returns to the state of starting transmission (S412) and continues transmitting information.
With the above-described retransmission scheme, channels have to be provided between the transmission side and the receiving side to transmit ACK/NACK acknowledgment information in opposite directions. When the user is on the transmission side and the base station is on the receiving side, channels between the base station and users are provided to transmit feedback information. In general, feedback information ACK/NACK is indicated by one or more bits.
With the conventional mobile communication network, the base station receives a large amount of data transmitted from mobile stations. After receiving the data, the base station has to transmit ACK/NACK information to each mobile station that transmitted data. Due to this large amount of acknowledgment information, a large amount of radio resources are consumed. Radio resources are being more valuable day by day, and how to make effective use of the resources is an important problem.